If only wheels could speak; if only we would listen. They might tell of the challenging life they lead, subject to constantly changing dynamics, twisting and deflecting due to rider and terrain. Assuming that’s the case both on and off-road, what is the optimal material for a bicycle rim? As in most things, the answer isn’t black and white. Here’s a short primer that identify some strengths and shortcomings for each material. I will separate my observations into road and off road usage.

Road.

Carbon structures are lighter than alloy, allowing manufacturers to produce deeper rims that weigh the same as a shallower alloy rim; a typical 45mm deep carbon rim weighs the same as a 27mm deep rim made from aluminum. The rider benefits from better aerodynamics, with no weight penalty. If life begins at 25mph for you, it’s a valid consideration.

Carbon road rims tend to absorb shock better; I say tend to, because the latest crop of wider alloy rims are much improved in ALL criteria, including absorption of road shock. Nevertheless, the first thing my customers notice when bolting on a pair of 38mm deep carbon wheels is how plush the ride becomes.

Alloy rims are a valid choice for the following reason. The latest rims from Easton, Hed, Velocity and others offer lower rolling resistance owing to wider internal widths, an area where carbon is still playing catch-up. A modern, 25mm wide alloy rim possesses an in internal width of approximately 20mm. In comparison, a carbon rim with an external width of 27-28mm has an internal width of 17-18mm. It doesn’t sound like much, but that additional ten percent width endows your tire with a wider and shorter contact patch. This is one of the technical factors that allow for lower rolling resistance -- chalk one up for alloy rims. There’s another advantage that alloy rims possess, especially important for riders in hilly country. For those who “ride” their brakes on long descents, carbon clincher rims can be damaged by heat build-up. Higher end products, such as Enve and Reynolds, are better able to withstand delaminating due to heat, but I have, at one time or another, seen every rim on the market today damaged by braking on long descents. No exceptions. Alloy rims don’t suffer from this problem; or bikes with disc brakes, obviously.

Off-road.

Mountain bikers don’t need aerodynamics; they need strength. Carbon mountain bike rims are roughly 20-30 percent lighter than an alloy rim of the same dimensions – with greater rigidity and impact resistance. Lighter rims require less energy to get moving, spinning up faster. Imagine cleaning technical uphill sections on your favorite trail after switching to carbon -- riders often report riding a gear or two higher following such an upgrade. Because of carbon’s higher impact resistance you can run lower tire pressure and not worry too much about dinging your rims.

Being lighter, carbon wheels steer better because they generate less gyroscopic force, allowing the front wheel to turn more readily. If you’ve ever tried to hold your line on twisty, off-camber terrain, you’ll know this is a good thing.

So, does alloy hold any advantages? A small one, perhaps; and One Big One. On fast descents, the flexible (less stiff) nature of an alloy rim will allow it to “read” the trail better. Faster riders familiar with both materials report that suspension and braking somehow feel better with alloy rims.

The Big One? That would be the cost, for both mountain and road. Carbon rims are still significantly more expensive, although the gap is slowly narrowing. Owing to the fact that it requires up to seven hours of intensive, hands-on labor to make a carbon rim, aluminum will always hold an advantage on price.